Fogged,direct-positive silver halide emulsion containing nitron

ABSTRACT

THIS INVENTION RELATED TO DIRECT-POSITIVE, SILVER HALIDE, PHOTOGRAPHIC ELEMENTS. IN ONE ASPECT, AT LEAST ONE LAYER OF SAID ELEMENT CONTAINS NITRON. IN ANOTHER ASPECT, AN OVER-LAYER CONTAINING LIGHT-INSENSITIVE SILVER CHLORIDE IS USED TO PROVIDE MORE UNIFORM PHOTOGRAPHIC CHARACTERISTICS IN USED DEVELOPERS. IN STIFF ANOTHER ASPECT, IMPROVED DIRECT-POSITIVE SILVER HALIDE GRAINS ARE PREPARED BY REDUCING A PORTION OF THE WATER-SOLUBLE SILVER SALT BEFORE ADMIXTURE WITH THE WATER-SOLUBLE HALIDE TO PRECIPITATE THE SILVER HALIDE GRAINS.

United States Patent 3,679,424 FOGGED, DIRECT-POSITIVE SILVER HALIDEEMULSION CONTAINING NITRON Jean-France Leon Paul Barbier and Guy Renard,Vincennes, France, assignors to Eastman Kodak Company, Rochester, N.Y.N0 Drawing. Filed Nov. 5, 1969, Ser. No. 874,393 Int. Cl. G030 1/34 US.Cl. 96-109 7 Claims ABSTRACT OF THE DISCLOSURE This invention relates todirect-positive, silver halide, photographic elements. In one aspect, atleast one layer of said element contains nitron. In another aspect, anover-layer containing light-insensitive silver chloride is used toprovide more uniform photographic characteristics in used developers. Instill another aspect, improved direct-positive silver halide grains areprepared by reducing a portion of the water-soluble silver salt beforeadmixture with the water-soluble halide to precipitate the silver halidegrains.

This invention relates to direct-positive, fogged silver halideemulsions. In one aspect, this invention relates to direct-positivesilver halide elements comprising at least one layer which containsnitron. In another aspect, this invention relates to a direct-positivephotographic element which comprises at least one direct-positive silverhalide emulsion layer and at least an overcoat layer and/or aninterlayer in said element which contains nitron. In still anotheraspect, this invention relates to a process for fogging direct-positiveemulsions and the products produced thereby.

It is known in the prior art to make direct-positive, fogged silverhalide emulsions. Emulsions of this type can be used in photographicelements to provide positive images upon exposure and chemicaldevelopment. In many of the prior direct-positive elements, imagecharacteristics upon development were highly dependent upon developmentconditions, length of use of the developer solutions, etc. The formationof a yellow fog is especially prevalent when the developing solution hasbeen extensively used. Density and contrast changes also occur with useddevelopers as opposed to fresh developing solutions. Therefore, improveddirect-positive photographic elements and processes for makingdirect-positive emulsions would be desirable to provide direct-positiveelements which demonstrate better image properties under a wide varietyof exposure and development conditions.

We have now found that fogged, direct-positive silver halide emulsionscan be made with improved photographic properties. In one aspect, wehave found that nitron can be used in overcoats, gelatin interlayersand/ or silver halide emulsion layers in direct-positive elements toprevent the formation of yellow physical development fog. In anotheraspect, we have found that direct-positive photographic elementscomprising fogged silver halide grains and at least one layer containingnitron can be processed after exposure in either fresh developer orextensively used developer to obtain substantially uniform sensitometriccharacteristics in the photographic element. In another aspect, we havefound that a silver chloride overlayer which preferably contains anexcess of chloride ions on the direct-positive element provides moreuniform image properties without substantial effects due to length ofuse of the developer, such as occurs when bromine ions build up in thedeveloper. In still another aspect of this invention, we have found thata portion of the silver nitrate used in the precipitation of thedirect-positive silver 3,679,424 Patented July 25, 1972 "ice halide canbe reduced with a reduction agent such as thiourea dioxide, stannouschloride and the like to provide improved direct-positive emulsions.

In a preferred embodiment, the direct-positive emulsions used incombination with layers which contain nitron aredirect-positive-blue-sensitive, chemically fogged silver halideemulsions.

In another preferred embodiment, the direct-positive emulsions comprisehalogen-accepting compounds.

In another preferred embodiment, the direct-positive emulsions compriseelectron-conducting compounds which are sometimes referred to asdesensitizers.

In one preferred embodiment of this invention, the direct-positivephotographic elements comprise layers which contain nitron. Generally,the nitron is utilized at a concentration of about milligrams to about 2grams per mole of silver halide in the element and preferably from about300 milligrams to about 1 gram. Nitron is generally known in the tradeto be l,4-diphenyl-3,5-endoanilino-4,5-dihydro-1,2,4-triazole or3,5,6-tripheny1-2,3,5, 6-tetraazabicyclo[2.l.1]hex1-ene.

The preferred direct-positive silver halide emulsions of this inventionare blue-sensitive. It is understood that bluesensitive means that thedirect-positive composition will provide a reversal image when exposedwith light in the 350- to SOO-millimicron range of the electromagneticspectrum. The silver halide compositions can also be spectrallysensitized so as to form reversal images when exposed in other regionsof the spectrum such as the red and green regions. However, they allhave the property of being capable of forming a reversal image whenexposed with light in the blue region of the spectrum. Generally, theseemulsions have high photographic speed compared to compositions such asconventional Hershel reversal emulsions.

Typical blue-sensitive-direct-positive silver halide emulsions which canbe used in combination with nitron in photographic elements withimproved properties include those disclosed in Berriman, US. Pat.3,367,778, issued Feb. 6, 1968; Illingsworth, Belgian Pats. 695,355through 695,366, all of which were granted Sept. 11, 1967; and Allentoffand Fogler, Belgian Pat. 689,233, granted Jan. 13, 1967. In oneembodiment, the silver halide emulsions can comprise silver halide graincenters which promote the deposition of photolytic silver. In oneembodiment of the invention, the sites for deposition of photolyticsilver are provided by reducing a portion of the Water-soluble silversalt such as silver nitrate before admixture in the reaction vessel withthe water-soluble halide such as, for example, potassium bromide.Reduction of the silver nitrate can take place with any common reducingagent such as, for example, stannous chloride, thiourea dioxide,formalin, alkaline arsenite and the like. In this embodiment, it is alsoadvantageous to add small amounts of polyvinyl pyrrolidone' to thesilver nitrate before reduction to obtain high contrast and maximumdensity with a small silver halide coverage ratio.

Typical direct-positive silver halide compositions which can becharacterized by the above definitions and which are useful in thisinvention are: (1) emulsions comprising silver halide grains havinginternal centers which promote the deposition of photolytic silver andan outer region or shell of a fogged insoluble silver salt andpreferably a halogen-conducting compound in said emulsion or (2) anemulsion which comprises fogged silver halide grains and an organiccompound which accepts electrons, said grains being such that a testportion thereof, when coated as a photorgraphic silver halide emulsionon a support to give a maximum density of at least about 0.5 uponprocessing for 5 minutes at about 68 F. in Developer A (formula at endof specification), has a maximum density which is at least about 30%greater than the maximum density of an identical coated test portionwhich is processed for 6 minutes at about 68 F. in Developer A afterbeing bleached for about minutes at about 68 F. in a bleach compositionof:

Potassium cyanide-50 mg. Acetic acid (glacia1)-3 .47 cc. Sodiumacetate11.49 g. Potasium bromide-119 mg. Water to 1 liter This inventioncan be practiced with direct-positive emulsions of the type in which asilver halide grain has a water-insoluble silver salt center and anouter shell composed of a fogged water-insoluble silver salt thatdevelops to silver without exposure. These emulsions can be prepared invarious ways, such as those described in Berriman, US. Pat. 3,367,778,issued Feb. 6, 1968. For example, the shell of the grains in suchemulsions may be prepared by precipitating over the core grains alight-sensitive, water-insoluble silver salt that can be fogged andwhich fog is removable by bleaching. The ,shell is of sufficientthickness to prevent access of 1 the developer used in processing theemulsions of the invention to the core. The silver salt shell is surfacefogged to make it developable to metallic silver with conventionalsurface image developing compositions. The silver salt of the shell issutficiently fogged to produce a density of at least about 0.5 whendeveloped for 6 minutes at 68 F. in Developer B below when the emulsionis coated at a silver coverage of 100 mg. per square foot. Such foggingcan be affected by chemically sensitim'ng to fog with the sensitizingagents described for chemically sensitizing the core emulsion,high-intensity light and the like fogging means well-known to thoseskilled in the art. While the core need not be sensitized to fog, theshell is fogged. Fogging by means of a reduction sensitizer, a noblemetal salt such as gold salt plus a reduction sensitizer, high pH andlow pAg silver halide precipitating conditions, and the like can besuitably utilized. The shell portion of the subject grains can also becoated prior to fogging.

Developer B N-methyl-p-aminophenol sulfate-2.5 g. Ascorbic acidl0.0 g.

Potassium met-aborate35.0 g. Potassium bromide-1.0 g.

Water to 1 liter pH of 9.6

Before the shell of water-insoluble silver salt is added to the silversalt core, the core emulsion is first chemically or physically treatedby methods previously described in the prior art to product centerswhich promote the deposition of photolytic silver, i.e., latent imagenucleating centers. Such centers can be obtained by various techniquesas described in the Berriman patent referred to above. I

Silver salt cores containing centers attributable to a metal of GroupVIII of the Periodic Table, e.g., palladium, iridium or platinum and thelike, are especially useful since these centers also appear to functionas electron acceptors. Chemical sensitization techniques of the typedescribed by Antoine Hautot and Henri Saubenier in Science et IndustriesPhotographiques, vol. XXVIII, January 1957, pages 1 to 23, and January1957, pages 57 to 65, are particularly useful. Such chemicalsensitization includes three major classes, namely, gold or noble metalsensitization, sulfur sensitization, such as by a labile sulfurcompound, and reduction sensitization, e.g., treatment of the silverhalide with a strong reducing agent which introduces small specks ofmetallic silver into the silver salt crystal or grain.

In another embodiment, the silver halide emulsions can comprise silverhalide grains having centers which promote the deposition of photolyticsilver which are either sufliciently small or sufiiciently buried withinthe crystal as to be not accessible to initiate development to a visibleimage. Silver halide grains of this type can be provided by either usingvery low concentrations of the sensitizing agent throughout theprecipitation or adding the sensitizing agent to the precipitationmedium during the initial part of the precipitation whereby theconcentration of the sensitizing agent will be lowered significantly byocclusion of the agent in the grains so that continued precipitationwould result in lowered concentration of centers for promotingdeposition of photolytic silver in the outer regions of each grain.

The practiceof this invention is particularly suit-able for high-speeddirect-positive emulsions comprising fogged silver halide grains and acompound which accepts electrons, as described and claimed inIllingsworth, patent application Ser. No. 619,909 and titledPhotographic Reve sal Materials III now US. Pat. 3,501,306, issued Mar.17, 1970. The fogged silver halide grains of such emulsions are suchthat a test portion thereof, when coated as a photographic silver halideemulsion on a support to give a maximum density of at least about 1 uponprocessing for 6 minutes at about 68 F. in Developer A, has a maximumdensity which is at least about 30% greater than the maximum density ofan identical coated test portion which is processed for 6 minutes atabout 68 F. in Developer A after being bleached for about 10 minutes atabout 68 F. in a bleach composition of:

Potassium cyanide-50 mg. Acetic acid (glacial)3.47 cc. Sodiumacetate-11.49 g. Potassium bromide-119 mg. Water to 1 liter The grainsof such emulsions will lose at least about 25% and generally at leastabout 40% of their fog when bleached for 10 minutes at 68 F. in apotassium cyanide bleach composition asdescribed herein. This fog losscan be illustrated by coating the silver halide grains as a photographicsilvver halide emulsion on a support to give a maximum density of atleast 1.0 upon processing for 6 minutes at about 68 F. in Developer Aand comparing the density of such a coating with an identical coatingwhich is processed for 6 minutes at 68 F. in Developer A after beingbleached for about 10 minutes at 68 F. in the potassium cyanide bleachcomposition. As already indicated, the maximum density of the unbleachedcoating will be at least 30% greater, generally at least 60% greater,than the maximum density of the bleached coatmg.

The silver halides employed in the preparation of the photographicemulsions useful in this invention include any of the photographicsilver halides as exemplified by silver chloride, silver bromide, silverbromoiodide, silver chlorobromide, silver chlorobromoiodide, and thelike. Emulsion blends, e.g., blends of silver chloride and silverchlorobromide, can be used. Also, the core of the silver halide graincan be composed of silver halide of diflferent composition than that inthe outer shell of the grain.

Silver halide grains having an average grain size less than about 2microns, preferably less than about 0.5 micron, give particularly goodresults. The silver halide grains can be regular and can be any suitableshape such as cubic or octahedral, as described and claimed inIllingsworth, patent application Ser. No. 619,948, and titledDirect-Positive Photographic Emulsions I, now US. Pat. 3,501,305, issuedMar. 17, 1970. Such grains advantageously have a rather uniform sizefrequency distribution, as described and claimed in Illingsworth, patentapplication Ser. No. 619,936, titled Photogaphic Reversal Emulsions II,now US. Pat. 3,501,307, issued Mar. 17, 1970. For example, at least byweight, of the photographic silver halide grains are within about 40%,preferably within about 30%, of the mean grain size. Average grain sizecan be determined using conventional methods, e.g., as shown in anarticle by Trivelli and Smith entitled Empirical Relations BetweenSensitometric and Size-Frequency Characteristics in PhotographicEmulsion Series in The Photographic Journal, vol. LXXIX, 1949, pages330-338, and Methods of Particle-Size Analysis, ASTM Symposium on LightMicroscopy, by '-Loveland, 1953, pages 94-122. The fogged silver halidegrains in these direct-positive photographic emulsions of this inventionproduce a density of at least 0.5 when developed without exposure forminutes at 68 F. in Developer A when such an emulsion is coated at acoverage of 50 to about 500 mg. of silver per square foot of support.The photographic silver halides can be coated at silver coverages in therange of about 50 to 500 milligrams of silver per square foot.

In preferred embodiments of this invention, electron acceptors andhalogen conductors (sometimes referred to as halogen acceptors) arepresent in the direct-positive emulsions.

The electron acceptors or halogen conductors which give particularlygood results in the practice of this invention can be characterized interms of their polarographic halfwave potentials, i.e., their oxidationreduc tion potentials determined by polarography. The electron acceptorsuseful herein have an anodic polarographic potential and a cathodicpolarographic potential which, when added together, give a positive sum.The halogen conductors useful herein have an anodic polarographicpotential less than 0.85 and a cathodic polarographic potential which ismore negative than l.0. Preferred halogen conductors have an anodicpolarographic potential less than 0.62 and a cathodic polarographicpotential which is more negative than l.3. Cathodic measurements can bemade with a 1X10- molar solution of the electron acceptor in a solvent,for example, methanol which is 0.05 molar in lithium chloride using adropping mercury electrode with the polarographic half-wave potentialfor the most positive cathodic wave being designated 03,. Anodicmeasurements can be made with 1X10" molar aqueous solvent solution, forexample, methanolic solutions of the electron acceptor which are 0.05molar in sodium acetate and 0.005 molar in acetic acid using a carbonpaste of pyrolytic graphite electrode, with the voltammetric half peakpotential for the most negative anodic response being designated 13,. Ineach measurement, the reference electrode can be an aqueous silversilverchloride (saturated potassium chloride) electrode at 20 "C.Electrochemical measurements of this type are known in the art and aredescribed in New Instrumental Methods in Electrochemistry, by Delahay,Interscience Publishers, New York, 1954; Polarography, by Kolthoff andLingane, 2nd edition, Interscience Publishers, New York, N.Y., 1952;Analytical Chemistry, 36, 2426 (1964) by Elving; and AnalyticalChemistry, 30, 1576 (-1958) by Adams. Signs are given according toIUPAC, :Stockholrn Convention 1953.

Advantageously, these electron acceptors used herein also providespectral sensitization such that the ratio of minus blue relative speedto blue relative speed of the emulsion is greater than 7, and preferablygreater than 10, when exposed to a tungsten light source through WrattenNo. 16 and No. 35 plus 38A filters respectively. Such electron acceptorscan be termed spectrally sensitizing electron acceptors. However,electron acceptors can be used which do not spectrally sensitize theemultAIl especially useful class of electron acceptors which can be usedin the direct-positive photographic silver halide emulsions andprocesses of this invention are cyanine dyes, such as theimidazo[4,5-b]quinoxaline dyes. Dyes of this class are described inBrooker and Van Lare Belgian Pat. 660,253, issued Mar. 15, 1965. Inthese dyes, the imidazo[4,5-b]quinoxaline nucleus is attached, throughthe 2-carbon atom thereof, to the methine chain. Typical goodelectron-acceptor dyes used in direct-positive emulsions are disclosedin Illingsworth and Spencer, Belgian 'Pat. 695,364, granted Sept. 11,1967.

A preferred class of halogen-conducting compounds useful in thisinvention is characterized by an anodic halfwave potential which is lessthan 0.62 and a cathodic halfwave potential which is more negative than1.3. A preferred class of halogen conductors that can be used in thepractice of this invention comprises the spectral sensitizingmerocyanine dyes having the formula:

0 II I l3 LL ..d A'1 where .A represents the atoms necessary to completean acid heterocyclic nucleus, e.g., rhodanine, 2-thiohydrantoin and thelike, B represents the atoms necessary to complete a basicnitrogen-containing heterocyclic nulceus, e.g., benzothiazole,naphthothiazole, benzoxazole and the like, each L represents a methinelinkage, e.g., -OH=,

and n is an integer from 0 to 2, i.e., 0, 1 or 2. Typicalhalogen-conducting compounds are disclosed in 'Wise, Belgian Pat.695,361, granted Sept. 11, 1967.

In the preparation of the above photographic emulsions, the electronacceptors, halogen conductors, bromide and iodide salts areadvantageously incorporated in the washed, finished silver halideemulsion and should, of course, be uniformly distributed throughout theemulsion. The methods of incorporating such addenda in emulsions arerelatively simple and well-known to those skilled in the art of emulsionmaking. For example, it is convenient to add them from solutions inappropriate solvents, in which case the solvent selected should becompletely free from any deleterious effect on the ultimatelight-sensitive materials. Methanol, isopropanol, pyridine, water, etc.,alone or in admixtures, have proven satisfactory as solvents for theelectron acceptors and halogen conductors. The type of silver halideemulsions that can be sensitized with these dyes include any of thoseprepared with hydrophilic colloids that are known to be satisfactory fordispersing silver halides, for example, emulsions comprising naturalmaterials such as gelatin, albumin, agar-agar, gum arabic, alginic acid,etc., and hydrophilic synthetic resins such as polyvinyl alcohol,polyvinyl pyrrolidone, cellulose ethers, partially hydrolyzed celluloseacetate, and the like. The binding agents for the emulsion layer canalso contain dispersed polymerized vinyl compounds such as disclosed,for example, in US. Pats. 3,142,568, 3,193,386, 3,062,674 and 3,220,-844, and include the water-insoluble polymers of alkyl acrylates andmethacrylates, acrylic acid, sulfoalkyl acrylates or methacrylates andthe like.

The overlayers are generally very effective in maintaining constantreproducibility of image properties with used developers when silverhalide direct-positive emulsions are used in the element wherein apredominant amount of the halide is bromide. The overlayer preferablycontains chloride ions or silver chloride, but generally any silvercompounds more soluble than silver bromide including silverferrocyanides can be utilized as they will be displaced by bromide. Whenthe overlayer is a silver compound, it is preferably desensitized tolight. Typical desensitizers useful for this purpose include rhodiumammonium chloride, 1-phenyl-5-mercaptotetrazole, methyl benzotriazole,4-nitro-6-chlorobenzotriazole and the like. 'Useful concentrations ofthese desensitizers are from about 0.1 to 0.5 percent per mole ofsilver.

The overlayers and silver halide layers of this invention can behardened by any acceptable means known in the photographic art; however,aldehyde hardeners such as formaldehyde and mucochloric acid arepreferred. The overlayers can be coated at any suitable concentration toprovide the necessary stability in image properties, but they arepreferably coated at about 7 to 7 about 15 mg./dm. of the ingredient tobe reduced by the bromide ions.

This invention can be further illustrated by the following examples.

EXAMPLE 1 A silver halide direct-positive emulsion is prepared by addingan aqueous solution containing 170 g. of silver nitrate which have beenpartially reduced to an aqueous solution containing .131 g. of potassiumbromide and 2.25 g. of potassium iodide. The precipitate is decanted,washed, gelatin added, chilled and set. This emulsion has fog specksdistributed throughout the grains as a result of the partial reductionof the silver nitrate.

To separate aliquot portions of the emulsion are added 0.127 g. ofdiphenylamino--[-(3 ethyl 2(3H) benzoxasolylidene)-ethylene]-5-isothiohydantoin (also known as 2diphenylamino-S-H3-ethyl-2-benzoxazolinylidene)ethylidene]-2-thiazolin-4-one) in methanol solution. A solutioncontaining .0018 g. of potassium chloroaurate, a gelatin solution and amucochlon'c acid solution are also added. To one of the portions isadded an aqueous solution containing .0012 g. of nitron.

The emulsions are coated on a support and dried. Respective samples ofthe emulsions are then exposed in a sensitometer and developed in ahydroquinone-l-phenyl- 3-pyrazolidone developer containing sodiumbisulfite and methoxy polyethylene glycol.

The samples containing the nitron contain no noticeable yellow physicaldevelopment t'og, while the control sample containing no nitron containssubstantial yellow physical development fog.

Samples of the exposed control emulsion and the emulsion containingnitron are then developed in the samples of the above developer whichcontains respectively 8 g., 14 g., 18 g. and 28 g. of sodium bromide perliter to approximate conditions of using extensively used developers.When the amount of bromide is increasing, the ED of nitron-containingsamples is slightly decreasing, whereas in control samples withoutnitron, 13 gamma and yellow fog are highly increasing.

It is apparent that the nitron represses the solvent action ofextensively used developers which contain higher concentrations ofbromide ions. 2

Similar results are obtained when nitron is used in an overlayer on theemulsion or in a gelatin interlayer within the layer arrangement.

Similar results are also obtained when the dyes diphenylamino 5-(3-ethyl-2(3H)-benzothiazolylidene) ethylene]- S-isothiohydantoin and[3-ethyl-2(3H)-benzoxaoylidene ethylidene] 3phenyl-l-carboxymethyl-Z-thiohydanfoin (also known as1-carboxymethyl-5-[(3-ethyl-2-benzoxazolinylidene) ethylidene] 3phenyl-Z-thiohydantoin) are utilized in the emulsion.

EXAMPLE 2 A fogged silver halide, direct-positive emulsion containingsilver halide grains having silver bromide outer layers or shells isprepared according to Allentofl and Fogler, U.S. Ser. No. 582,262, filedSept. 27, 1966, now U.S. Pat. 3,477,852, issued Nov. 11, 1969. To oneportion of the emulsion is added 650 milligrams per mole of silver ofnitron. Another portion served as a control.

Samples of the coated emulsion are exposed on a sensitometer anddeveloped for 2% minutes in Kodak D-85 developer containing 1 g. of hypoper liter. The control sample produces a high degree of yellow fog,whereas the sample containing nitron has no apparent yellow fog.

The background density is measured through a blue filter to give anindication of yellow stain. The control sample has a density of 0.15while the sample containing 650 mg. of nitron per mole of silver has adensity of 0.10.

EXAMPLE 3 A fogged direct-positive silver chlorobromide gelatin emulsionis prepared as described in Example 5 of Litzer- 8 man, U.S. Ser. No.618,354, filed Feb. 24, 1967, now U.S. Pat. 3,531,290, issued Sept. 29,1970, containing g. of gelatin/mole of silver halide. To this emulsionare added 80 g. of copoly(methyl acrylate-sulfopropyl aerylate-Z-aceto-acetoxy ethyl methacrylate); 500 mg. of a halogen acceptor dye,3-ethyl-5-[1-(4-sulfobutyl)4-( lH)-pyridylidene]-rhodanine sodium salt;and 1 g. of potassium iodide/ mole of silver halide. Formaldehyde isadded as a hardener, and part of the emulsion is coated on apolyethylene terephthalate support. To a second part of the emulsion,1.5 g. of nitron is added and the emulsion coated on a similar support.Both emulsions are coated at a coverage of 450 mg. of silver and 300 mg.of gelatin/ft. A sample of each coating is exposed on an intensity scalesensitometer and processed for 1% minutes in an amine developer of thetype described in Example 1 of Masseth, U.S. application Ser. No.661,532 (Developer A), now U.S. Pat. 3,573,914, issued Apr. 6, 1971,fixed, washed and dried using a roller transport continuous processingmachine. The following results are obtained.

A silver chloride emulsion is made by mixing, over a period of 1 minuteat 35 C., a gelatin solution containing g. of silver nitrate with anaqueous gelatin solution containing g. of sodium chloride and 0.01 g. ofa mixture of rhodium and ammonium chloride. The emulsion is neutralizedto a pH of 5.6 and a phthalated gelatin solution is added. The emulsionis coagulated by lowering the pH, then decanted and taken up in agelatin solution. Mucochloric acid (0.5 g.), nitron ('1 g.) andl-phenyl-5- mercaptotetrazole (0.1 g.) are added to the emulsion.

The above chloride emulsion is then coated at a coverage of 10 mg. ofsilver per drn. over an X-ray duplicating direct-positive emulsion whichhas a gelatin overlayer.

The X-ray duplicating film having a fogged direct-positive emulsion isprocessed in a developer containing various concentrations of bromideion simulating the buildup of bromide during extensive processing.

The maximum image density of the elements having chloride overcoats ismuch more stable and the relative speed and gamma of the photographicelement is much more stable as compared with the element without thechloride overcoat.

Similar results are obtained when silver ferricyanide is used in theoverlayer in place of silver chloride.

As used herein, and in the appended claims, fogged refers to emulsionscontaining silver halide grains which produce a density of at least 0.5when developed, without exposure, for 5 minutes at 68 F. in Developer Ahaving the composition set forth below, when the emulsion is coated at asilver coverage of 50 mg. to 500 mg. per square Water to make 1.0 liter.

Although the invention has been described in considerable detail withparticular reference to certain prefererd embodiments thereof,variations and modifications can be effected within the spirit and scopeof the invention.

We claim:

1. -A direct-positive photographic element which comprises a supporthaving thereon at least one layer containing a fogged direct-positivesilver halide emulsion and at least one layer containing 100 mg. toabout 2 g. of. nitron per mole of silver in said element.

2. A direct-positive element according to claim 1 wherein saiddirect-positive silver halide emulsion layer contains nitron.

3. A direct-positive element according to claim 1 wherein the silvernitrate used to make said direct-positive silver halide emulsion ispartially reduced prior to precipitating said silver halide.

4. A direct-positive element according to claim 1 wherein saiddirect-positive emulsion is a blue-sensitive-directpositive silverhalide emulsion containing silver halide grains which have beenchemically fogged.

5. A direct-positive element according to claim 1 wherein the halide ofsaid silver halide is predominantly bromide and wherein said elementcomprises an overlayer which contains chloride ions, silver chloride orsilver ferrocyanides the ions of which are displaced by bromide ions.

6. A fogged, direct-positive photographic element which comprises asupport having thereon at least one layer containing a fogged,direct-positive silver halide emulsion and at least one layer containingabout mg. to about 1 g. of nitron per mole of silver in said element.

7. A direct-positive element according to claim 1 wherein the halide ofsaid silver halide is predominantly bromide and wherein said elementcomprises an overlayer which contains chloride ions, or a silvercompound more soluble than silver bromide having ions which aredisplaceable by bromide ions.

References Cited UNITED STATES PATENTS 2,005,837 6/1935 Arens 96-64 X2,500,140 3/ 1950 Teal et a1 96-95 3,297,446 I/ 1967 Dunn 96-107 NORMANG. TORCHIN, Primary Examiner W. H. LOUIE, JR., Assistant Examiner US.Cl. X.-R. 96-64, 107

